The present invention is directed to a process for fusing images generated by electrophotographic methods, developed, and transferred to a substrate. More specifically, the present invention is directed to a process for fusing electrophotographic images which comprises applying heat and/or pressure to a transferred image with a fuser roll having a surface of an insulating material to which has been applied a charge of the same polarity of the toner particles with which the image was developed, thereby assisting in the release of toner from the fuser roll and reducing or eliminating offset without the need for use of materials such as release oils. Offset occurs when, during the fusing process, toner adheres to the fuser roll, which impairs image quality. In one embodiment of the present invention, the fuser roll is charged to the same polarity as the toner by means of a corotron, which results in the fuser roll repelling the toner particles. In another embodiment of the invention, the fuser roll surface comprises a polymeric material having embedded therein charges of the same polarity as that of the toner. In yet another embodiment of the invention, the fuser roll comprises a resistive material, which enables generation of both heat and surface charge upon application of voltage to the surface of the fuser roll.
Fusing processes for electrophotographic images are known. For example, U.S. Pat. No. 3,740,249 discloses a solvent fixing process which comprises contacting a film of a solvent formed on the surface of a grounded conductive roller with the toned images. A corona discharge of polarity opposite to that of the toner is applied to the back of the paper, which draws the toner away from the fuser roll, and the nonconductive, nonpolar solvent fixes the toner image to the paper. The corona discharge brings the sheet into close contact with the roller to provide uniform fixing and also attracts the toner to the paper, which reduces offset onto the roller and reduces disturbance of the image by the solvent. Alternatively, instead of charging the paper by corona discharge, the paper may be passed through two rollers, one of which is biased to a polarity opposite to that of the toner.
In addition, U.S. Pat. No. 4,320,714 discloses a heat fixing device, wherein an electroconductive layer is provided in the surface of one of the fixing and pressure rolls to prevent chage buildup on the roll surface. Preferably, the layer is aluminum plated and covered with a nonadhesive layer such as tetrafluoroethylene, or HTV or RTV silicone rubber. The fuser roll prevents an electric field from forming and extending through an imaged sheet to be fixed, and eliminates offset on the roll. The pressure roller contains a thin grounded metal layer as near to the outside edge of the roll as possible, and a grounded electrode reverses the direction of the electrostatic field caused by spurious charge buildup on the roll.
Further, U.S. Pat. No. 3,893,800 discloses an apparatus for fusing electrophotographic images wherein a contact fuser softens the powder images by means of heat conducted through the back of the substrate. Heat is applied to the back side of the substrate by a heated roll to which is electrostatically tacked the substrate in order to improve heat transfer thereto. Trail-end flip up of the substrate is prevented by producing an attractive force between the substrate and the means for guiding the substrate to the heated roll after the substrate has been tacked to the heated roll.
Although known processes for fusing electrophotographic images are suiable for their intended purposes, a need continues to exist for fusing processes that reduce or eliminate offset without the need for release oils, since release oils constitute an added expense, lead to problems caused by the presence of fuser oil on fused copies, may interact unfavorably with photoreceptor materials, and lead to customer inconvenience. A need also exists for fusing processes that enable fusing of electrophotographic images without the need for release oils and that enable formation of high quality images.